One of the major issues in the telecommunications industry today is the ongoing demand for more and more bandwidth. Today, so-called third generation networks employ Wavelength Division Multiplexing technology where both the transmission and the switching of data are in the optical domain. Dense Wavelength Division Multiplexing (DWDM) involves the process of multiplexing many different wavelength signals onto a single fibre. Use of DWDM allows providers to offer services such as e-mail, video, and multimedia carried as Internet Protocol (IP) data over asynchronous transfer mode (ATM) and voice carried out Synchronous Optical NETwork (SONET) (or Synchronous Digital Hierarchy (SDH). SONET/SDH are defined by a set of related standards for synchronous data transmission over fibre optic networks. The standard for SONET is the United States version and is published by the American National Standards Institute (ANSI). The international version of SDH is the standard published by the International Telecommunications Union (ITU). The differences between SONET and SDH are slight and restricted to the basic frame format.
Despite the fact that these formats—IP, ATM, and SONET/SDH—provide unique bandwidth management capabilities, all three can be transported over the optical layer using DWDM. This unifying capability allows the service provider the flexibility to respond to differing customer demands over one network.
One property of a DWDM optical network is the ability to do wavelength routing. Here, the path of the signal through the network is determined by the wavelength and origin of the signal, as well as the states of the network cross-connects and wavelength changers. Wavelength routing provides a transparent light path between network terminals. A light path is the path that an optical signal traverses in the network from a source to a single destination and may include all-optical wavelength changers.
A property of optical cross-connects is that the optical channels, (also referred to as wavelengths or colours) which are typically fully utilised in carrying data and the related protocols, can be transmitted and inter-connected without knowledge of the data protocol, or even the bit-rate of the data.
There exist cross-connects (including switches, multiplexors, concentrators and interconnects) which need have no knowledge of the data or protocol. These cross-connects act purely at the ‘physical layer’, the Layer 1 of the International Standards Organisation (ISO) protocol stack. A number of such cross-connects may be co-located to permit higher concentration of traffic thereby taking advantage of the inherent high bandwidth of DWDM transmission systems.
The very nature of the optical path, carrying unknown bit-rate data of unknown protocol, presents problems in managing optical networks. In particular, if a certain cross-connect element, such as cross-point, is introducing errors into the data being carried it is very difficult to determine that it is not functioning correctly so that corrective measures can be taken. Previously, various techniques have been used in attempts to improve the situation. For example, connection integrity monitoring has been used to ensure a signal is connected to the correct port of a cross-connect, but this approach cannot determine whether it is routed correctly within the cross-connect system. Similarly, a monitor at the destination card has been used to determine if a signal is correct, but is unable to trace the signal through the cross-connect system.
What is needed is a cost-effective method for determining both the signal performance and connection integrity of each element of a cross-connect complex.